1. Field of the Invention
The present invention relates to a localization apparatus for recognizing a location of a node in a sensor network and a method thereof; and, more particularly, to a localization apparatus selectively using neighbor nodes having less obstacle factors and a method thereof.
2. Description of Related Art
Triangulation has been generally used to detect a location of a node in a wireless sensor network. The triangulation uses coordinates and distances of more than three anchor nodes to detect the location of a node. A localization method using triangulation is generally classified into a localization method using information about measured distances and a localization method not using information about measured distances.
In the method using distance information, a location of a target node is detected by measuring distances from more than three known anchor nodes to the target node and performing the triangulation with the measured distances and the location information of the known anchor nodes. A distance between two nodes is measured using a time of arrival (ToA) method, a time difference of arrival (TDoA) method, a received signal strength (RSS) method.
In the ToA method, a distance between two nodes is measured using a time taken by a signal having a known propagation speed to travel between two nodes. In the TDoA method, two signals having different propagation speeds are simultaneously transmitted to two nodes, and a distance between two nodes is measured using the arrival times of two signals. The two methods may provide more accurate measuring results if a signal has a low propagation speed and if no obstacles are present between two nodes. In case of the ToA method, it is impossible to accurately measure a distance if a signal has a fast propagation speed like a radio frequency (RF) signal. In case of the TDoA method, additional hardware or sensors are necessary to measure a distance between two nodes because two signals are used. In addition, it is impossible to obtain accurate distance measuring results if signals having a low propagation speed, such as an ultrasonic wave or a sound wave, are used. That is, it is difficult to secure line of sight (LoS) if an ultrasonic wave or a sound wave is used. Therefore, it is impossible to accurately measure a distance between two nodes due to the influence of obstacles.
In the RSS method, a distance is measure using the strength of a receiving signal arrived at a node. The RSS method uses a radio frequency (RF) signal. Since the RF signal has a better diffraction character than an ultrasonic wave or a sound wave, it is easier to secure line of sight (LoS) and additional hardware is not required. However, the RSS method has a disadvantage of low accuracy compared to other distance measuring methods. Particularly, the measuring result of the RSS method is significantly influenced by obstacles that reflect or absorb the RF signal.
As a localization method not using distance information, a Centroid method and an approximate point in triangulation (APIT) method were introduced. Such methods were developed because the localization method using distance information may diffuse an error in a sensor network having multi-hop. In the Centroid method, regularly arranged anchor nodes transmit the location information thereof to neighbor nodes, and the neighbor nodes estimate own location by comparing strength of signals received from the anchor nodes. The Centroid method may provide more accurate measuring result if the anchor nodes are arranged at a regular distance, if the number of anchor nodes is large, and if the RF propagation environment is identically sustained. However, it is very difficult to satisfy such conditions in a real environment of a sensor network, such as a general indoor place. In the APIT method, a node estimates an own location using whether the node is present in a triangle formed of anchor nodes. Since a node also uses signal strength to estimate own location in the APIT method, the APIT method has disadvantage identical to the Centroid method.
As described above, the localization methods using triangulation according to the related art needs the large number of anchor nodes and nodes to detect own locations to accurately detect a location of a node. If the number of anchor nodes increases, the number of triangles increases too. Therefore, the large amount of computation is required to accurately detect the location.